The present invention is related to hydrogen sensors, and more particularly, to a robust single-chip hydrogen sensor and method for manufacturing the same.
During the early 1990s, Sandia National Laboratory developed a single-chip hydrogen sensor that utilized Palladium-Nickel (PdNi) metal films as hydrogen gas sensors. U.S. Pat. No. 5,279,795, naming Robert C. Hughes and W. Kent Schubert as inventors, assigned to the United States as represented by the U.S. Department of Energy, describes such a sensor and is incorporated by reference herein.
One of the key benefits of the sensor described in the ""795 patent is its ability to detect a dynamic range of hydrogen concentrations over at least six orders of magnitude. Prior solutions to the problem of detecting hydrogen concentrations had been generally limited to detecting low concentrations of hydrogen. These solutions include such technologies as metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field-effect-transistors (FET), as well as palladium-gated diodes.
The hydrogen sensor described in the ""795 patent was a notable advance in hydrogen-sensing technology. It was, however, primarily limited to an experimental laboratory environment due to the difficulties encountered in manufacturing such a sensor.
In typical silicon fabrication facilities, metal films are first blanket-deposited across the entire wafer, and are subsequently patterned by an etch process. However, conventional etchants for PdNi also attack aluminum, which is normally present on the wafer surface as an interconnect metal before the PdNi film is deposited. Patterning the PDNI by etching would also attack the unprotected aluminum, destroying the sensor. Other non-conventional semiconductor fabrication techniques involving the use of a photoresistive material applied before the PdNi in a xe2x80x9clift-offxe2x80x9d process have produced very low yields in tests performed by the assignee of the present invention. Low yields in the production of semiconductor devices typically translates to difficulties in producing a commercializable product.
One solution to the above problems is described in U.S. patent application Ser. No. 09/729,147, titled xe2x80x9cRobust Single-Chip Hydrogen Sensor,xe2x80x9d assigned to Honeywell International Inc., and incorporated by reference herein. The technique disclosed is a lift-off process, in which an adhesion promoting layer, such as chromium, is provided to cause a sense-resistive layer, such as a PdNi layer, to adhere to an underlying base layer. As a result, during the lift-off process, there is a reduced likelihood of sensor portions being lifted off in conjunction with the portions actually intended to be removed. However, the use of chromium has been discovered to be prone to at least one disadvantage. The chromium has a tendency to affect the operation of hydrogen sensing transistors on the sensing chip. As a result, accuracy and/or sensing range may be affected.
It would thus be desirable to provide a robust single-chip hydrogen sensor that is capable of sensing hydrogen concentrations over a broad range, such as from approximately 1% to approximately 100% concentrations.
It would also be desirable for such a sensor to be efficiently manufacturable, so that costs are reduced and the sensor is producible in high enough yields to enable commercialization.
It would be desirable for such a sensor to provide measurement results that approximate or improve on the results from previous hydrogen sensors.
It would additionally be desirable to minimize sensor drift and to improve device-to-device and wafer-to-wafer repeatability.
In accordance with an illustrative embodiment of the present invention, some of the problems associated with manufacturing a robust hydrogen sensor are addressed.
According to a first embodiment, a silicon-based hydrogen sensor is provided. The sensor includes at least one hydrogen sensing portion composed of a first material and at least one interconnect metallization also composed of the first material. The first material is preferably, but need not be, a palladium nickel alloy. The interconnect metallization is preferably covered with an oxide or nitride to make the interconnect metallization inert. In a first aspect of this embodiment, the hydrogen sensing portion and the interconnect metallization are formed concurrently with one another. In a second aspect of the invention, the sensor further includes an underlying layer and at least one contact between the interconnect metallization and the underlying layer. The underlying layer may, for example, be composed primarily of silicon, and the contact may be a silicided contact.
In a second embodiment, a silicon-based hydrogen sensor includes at least one hydrogen-sensing portion and at least one interconnect metallization. The hydrogen-sensing portion is patterned by a deposition, mask, and etch process, and the at least one interconnect metallization is composed of a material that is resistant to the deposition, mask, and etch process used to pattern the hydrogen-sensing portion. For example, the at least one hydrogen sensing portion may be composed of a palladium nickel alloy and the at least one interconnect metallization may be composed of a material that is relatively impervious to the hydrogen-sensing etch process, such as gold.
In a third embodiment, a method for fabricating a silicon-based hydrogen sensor is provided. The method includes providing an interconnect metallization and a hydrogen sensing portion made of the same material. The material is preferably a palladium nickel alloy. The interconnect metallization is preferably covered with an oxide or nitride to make the interconnect metallization inert. In another aspect of the invention, the method further includes providing a silicided contact between the interconnect metallization and an underlying base portion. The silicided contact may be provided by masking the underlying base portion, etching a contact portion from the masked underlying base portion, depositing a contact material, masking the contact material, and sintering the contact material. Exemplary materials that may be used to provide the contact include cobalt, titanium, tungsten, platinum, tantalum, and molybdenum.
In a fourth embodiment, a method for fabricating a single-chip hydrogen sensing-device is provided. The method includes forming a silicided contact on an underlying base portion, depositing a hydrogen sensing material over the silicided contact and the underlying base portion, masking a pattern in the hydrogen sensing material, and etching the hydrogen sensing material to form a hydrogen sensing portion and an interconnect metallization portion. In another aspect, the method may further include annealing the hydrogen sensing material. The interconnect metallization is preferably covered with an oxide or nitride to make the interconnect metallization inert. Forming the silicided contact may include depositing, etching, and sintering a first material, such as cobalt, titanium, tungsten, platinum, tantalum, and molybdenum.